1. Field of the Invention
The present invention relates to a frequency conversion apparatus preferably used in a receiver or a transmitter which executes frequency conversion at least two times.
2. Description of the Related Art
FIG. 3 is a block schematic diagram of a conventional frequency conversion apparatus for, for example, a television signal transmitter. A metal case 51 is formed in a rectangular frame shape with side plates 51a and 51b opposing each other in one direction and side plates 51c and 51d opposing each other in the other direction. Three partitioned chambers 51h, 51i and 51j are formed in parallel with each other by being separated from each other by shield plates 51e to 51g, which are interposed between the side plates 51a and 51b in parallel with the side plate 51c. Further, partitioned chambers 51m, 51n, 51o and 51p are formed by being separated from each other by shield plates 51k and 51l which are disposed in a cross state in the region surrounded by the shield plate 51g and the side plates 51a, 51b and 51d. As a result, the partitioned chamber 51h is located adjacent to the side plate 51c and the partitioned chambers 51o and 51p are located adjacent to the side plate 51d. A printed circuit board (not shown) is disposed in each of the partitioned chambers and a circuit is formed on the printed circuit board.
A first mixer 52 and a first local oscillator 53 are disposed in the partitioned chamber 51h, and external signals, for example, intermediate frequency signals Si for television broadcasting images (for example, 45.75 MHz), are supplied to the first mixer 52. The first local oscillator 53 supplies oscillation signals of about 1254 MHz to the first mixer 52. As a result, the first mixer 52 outputs first intermediate frequency signals of about 1300 MHz.
A first-stage first intermediate frequency amplifier 54 and a first stage band-pass filter 55 are disposed in the partitioned chamber 51i adjacent to the partitioned chamber 51h. The first intermediate frequency signals are supplied to the first-stage first intermediate frequency amplifier 54 and then sent to the next partitioned chamber 51j through the first stage band-pass filter 55.
A second-stage first intermediate frequency amplifier 56 and a second stage band-pass filter 57 are disposed in the partitioned chamber 51j. The first intermediate frequency signals are amplified with the second-stage first intermediate frequency amplifier 56 and further sent to the next partitioned chamber 51m through the second stage band-pass filter 57.
The partitioned chamber 51m is located adjacent to the partitioned chamber 51n and these partitioned chambers 51m and 51n are located adjacent to the partitioned chamber 51j. The partitioned chamber 51p is located adjacent to the partitioned chamber 51m and the partitioned chamber 51o is located adjacent to the partitioned chamber 51n. The partitioned chamber 51p and the partitioned chamber 51o are located adjacent to the side plate 51d.
A second mixer 58 is disposed in the partitioned chamber 51m, a second local oscillator 59 is disposed in the partitioned chamber 51o, an oscillation signal amplifier 60 is disposed in the partitioned chamber 51n, and a second intermediate frequency amplifier 61 is disposed in the partitioned chamber 51p.
The first intermediate frequency signals having passed through the second stage band-pass filter 57 are supplied to the second mixer 58.
In contrast, the oscillation signals from the second local oscillator 59 are amplified with the oscillation signal amplifier 60 and supplied to the second mixer 58. The oscillation frequency of the second local oscillator 59 changes from about 1348 MHz to about 2305 MHz. Therefore, second intermediate frequency signals so having any frequency from about 50 MHz to about 1000 MHz are output from the second mixer 58, amplified with the second intermediate frequency amplifier 61 and supplied to the outside from the partitioned chamber 51p. The second intermediate frequency signals So are transmitted to subscribers as cable television signals through a CATV cable (not shown).
A shield cover (not shown) is attached to the metal case 51 to integrally cover each of the partitioned chambers. The end surfaces of the shield plate 51e and the like are in contact with the shield cover when the shield cover is attached to the metal case 51, whereby the respective partitioned chambers are shielded from each other.
Power is supplied to the first mixer 52, the first local oscillator 53, and the like disposed in the respective partitioned chambers through conductive patterns 62 (shown by thick lines in FIG. 3) formed on the printed circuit boards in the partitioned chambers.
Incidentally, while the two local oscillators 53 and 59 oscillate with a different frequency, they output a multiplicity of higher harmonics at the same time.
On the other hand, while the shield cover is attached to the metal case 51, if small gaps exist between the end surfaces of the shield plate 51e, and the like of the metal case 51 and the shield cover, the higher harmonics from, for example, the first local oscillator 53 disposed in the partitioned chamber 51h, enter the partitioned chamber 51m, in which the second mixer 58 is disposed, through the gaps of the three shield plates 51e, 51f and 51g.
Further, the oscillation signals from the second local oscillator 59 are also supplied to the second mixer 58 and the higher harmonics are also supplied thereto.
For example, when the second local oscillator 59 oscillates with a frequency of 1904 MHz to set the frequency of the second intermediate frequency signals to about 604 MHz, if the second higher harmonics (frequency: 2508 MHz) of the first local oscillator 53 are supplied to the second mixer 58, beat signals having a frequency which is the difference therebetween (2508-1904=604 MHz) are generated by the second mixer 58. The signals act as interference signals to the second intermediate frequency signals of 604 MHz.